Various articles of manufacture contain at least one component which is based upon an EPDM containing rubber composition. Sometimes it is desired that such EPDM rubber composition contain particulate reinforcing filler such as precipitated silica and exfoliated clay. Other particulate fillers for consideration are, for example, mica, talc, kaolin clay and calcium carbonate. Rubber reinforcing carbon black may also be included in the EPDM based rubber composition.
An example of such article of manufacture having a component containing a low unsaturation rubber such as EPDM rubber is a pneumatic rubber tire where said component is an outer rubber sidewall layer which is be subject to considerable flexing and scuffing as well as atmospheric aging.
Further examples of such articles of manufacture containing such low unsaturation rubber (the EPDM rubber) are various engineered products which rely upon a degree of atmospheric weathering resistance, such as for example, hoses, hose covers, window gaskets, weatherstrips, seals, roofing and insulation related products. For such products, particulate filler may comprise one or more of said mica, talc, kaolin clay and calcium carbonate.
Use of a low unsaturation EPDM based rubber composition has been proposed for such articles of manufacture, particularly for a tire having an outer, atmospherically exposed, sidewall outer layer, to resist its aging because of the resistance of EPDM rubber to atmospheric (e.g. ozone) weathering, primarily because of its low unsaturation (carbon-to-carbon double bond) content. For example, see U.S. Pat. No. 5,386,865. Phase mixing of a precipitated silica-containing EPDM rubber has proposed. For example, see U.S. Pat. No. 7,423,089.
Reinforcement of the EPDM/conjugated diene-based elastomer blend is considered herein to be challenged which relies upon use of a coupling agent to couple the reinforcement, such as for example precipitated silica, to the elastomers is made more difficult because of the significant reduction of availability of carbon-to-carbon double bond (low unsaturation) of the EPDM portion of the composition which tends to restrict the ability of a silica coupling agent to bond to the EPDM portion of the rubber composition.
Indeed, for homogeneous mixing of the EPDM elastomer, conjugated diene-based elastomer, precipitated silica reinforcement and silica coupling agent, the silica reinforcement of the EPDM elastomer portion of the rubber composition is made more difficult because of the low unsaturation of the EPDM thereby rendering it less susceptible to silica coupling, or bonding, of the silica reinforcement to the EPDM elastomer as compared to conjugated diene-based elastomers in the rubber composition such as, for example, cis 1,4-polyisoprene and cis 1,4-polybutadiene elastomers.
It is therefore proposed to evaluate preparation of a coupling agent enhanced reinforcement-containing rubber composition composed of both EPDM and conjugated diene-based elastomers by a phase mixing process.
For such phase mixing process, preparation of an EPDM-containing rubber composition is conducted in, for example, two phases.
In a first mixing phase, EPDM rubber, and particulate filler comprised of precipitated silica and optionally exfoliated clay platelets is mixed with,
(A) rubber reinforcing carbon black, or
(B) coloring pigment of a color other than black, without carbon black (where a colored rubber composition other than a black color is desired);
wherein said precipitated silica is provided:
(A) with a silica coupling agent, or
(B) without a silica coupling agent.
If desired, the precipitated silica is used, or provided, together with a silica coupling agent to enhance a rubber reinforcing effect of the precipitated silica to the low unsaturation-containing (low carbon-to-carbon double bond-containing) EPDM rubber.
For products which may be considered as not necessarily needing such degree of coupling of the precipitated silica to the EPDM rubber, the silica coupling agent might desirably not be included, such as example, products like various engineered products exemplary of which are, for example, various hoses, roofing, weather strips and window gaskets, the decision as to not including the silica coupling being made depending upon an individual product property desired.
Optionally, additional low unsaturation polymers, or polymers having a low, or non-existent, carbon-to-carbon double bond unsaturation, may be included with the EPDM rubber in the initial (first) mixing phase such as, for example, at least one of polyethylene, polypropylene, ethylene/propylene copolymers, styrene/ethylene/propylene terpolymers as well as copolymers and halogenated copolymers of paramethylstyrene and isobutylene.
In a second mixing phase, the rubber composition from the first mixing phase is mixed with at least one conjugated diene-based elastomer (e.g. combination of conjugated diene-based elastomers) selected from, for example, cis 1,4-polyisoprene rubber, cis 1,4-polybutadiene rubber and styrene/butadiene rubber, particularly a combination of cis 1,4-polyisoprene and cis 1,4-polybutadiene rubbers where a tire sidewall rubber composition is contemplated which may be subject to flexing and scuffing and exposed to atmospheric conditions such as, for example, ozone exposure.
For either or both of the first and second mixing phases, additional particulate reinforcing fillers, other than said precipitated silica, rubber reinforcing carbon black and exfoliated clay platelets, may be included, if appropriate and desired, such as at least one of mica, talc, kaolin clay, calcium carbonate. Such additional fillers are considered herein as being reinforcing fillers in a sense of providing what is considered herein as providing some degree, although a minor degree, of reinforcement for the elastomers.
By such separate, or segmented, mixing phases, the particulate filler added in the first mixing phase associates itself first, and thereby primarily, with the EPDM rubber, before contact with the conjugated diene-based elastomer(s) which are is (are) added in the second mixing phase.
For example, where precipitated silica is included as the particulate filler, together with silica coupling agent, in the first mixing phase, the silica coupling agent assists in selectively coupling the precipitated silica to the EPDM elastomer prior to contact with the conjugated diene-based elastomer which is added separately in the aforesaid second mixing phase.
In this manner, the coupled reinforcement is (becomes) more effectively (e.g. primarily) associated with the EPDM elastomer portion of the rubber composition in a sense of creating an enhanced filler reinforced EPDM-containing rubber composition, particularly insofar as the silica reinforcement aspect of the rubber composition is concerned. This is a significant aspect of the invention.
In order to facilitate such phase mixing of the rubber composition it is also proposed to evaluate an effect of significantly increasing the content of the coupling agent in the aforesaid silica and/or exfoliated clay platelet reinforcement-containing, low unsaturation-containing EPDM mixing phase, namely the first mixing phase, in order to promote preferential bonding of the particulate reinforcement of silica and/or exfoliated clay platelets to the EPDM.
Historically, in U.S. Pat. No. 7,423,089, a sulfur curable rubber composition is proposed for a tire sidewall which is composed of a combination of at least two diverse EPDM elastomers together with at least one conjugated diene-based elastomer, and reinforcing filler is comprised of at least one of carbon black, silica and exfoliated clay, where the rubber composition is prepared by a phase mixing process.
This invention is intended to differ significantly from such phase mixing process by a selective inclusion of a significantly high content of coupling agent in the silica reinforcement containing EPDM elastomer phase, namely the first mixing phase.
By such sequential phase mixing process, the particulate reinforcement of precipitated silica, which may optionally include exfoliated clay platelets, with the aid of the significant coupler content, remains primarily associated with the EPDM elastomers in the EPDM/conjugated diene-based elastomer (e.g. cis 1,4-polyisoprene and cis 1,4-polybutadiene elastomers) composition in the aforesaid first mixing phase and therefore promotes an enhancement of the overall reinforcement of the rubber composition with particulate reinforcement as at least one of precipitated silica and exfoliated clay platelets as compared to a more simple homogeneous mixing (instead of phase mixing) of the EPDM rubber and conjugated diene-based rubbers.
For the purposes of this invention, the EPDM rubber may be composed of at least two EPDM elastomers which are differentiated from each other.
For such purpose, a first EPDM elastomer is provided which is more elastomeric in nature by containing a higher non-conjugated diene content and thereby promotes a more rapid rate of sulfur curing, insofar as the EPDM elastomer is concerned, to aid in reducing the sulfur rate incompatibility between the EPDM rubber and conjugated diene-based elastomer(s), such as for example the natural cis 1,4-polyisoprene rubber, cis 1,4-polybutadiene and styrene/butadiene rubber.
An optional second EPDM elastomer is provided which is less elastomeric in nature than the first EPDM elastomer by having a lower non-conjugated diene content. Its somewhat higher ethylene content (e.g. therefore a higher ethylene/propylene ratio) may present the second EPDM elastomer as being somewhat being somewhat more self-reinforcing than the first EPDM elastomer in a sense of containing increased polyethylene-based crystalline domains because of its higher ethylene content levels.
The use of the dual EPDM elastomer blend, then, is intended to promote both mechanical strength for the rubber composition and interfacial strength between the EPDM rubbers and conjugated diene-based rubbers, particularly the conjugated-diene based rubbers comprised of cis 1,4-polyisoprene rubber, particularly natural cis 1,4-polyisoprene rubber.
In practice, the non-conjugated diene for the EPDM terpolymer rubber may be selected from, or comprised of, for example, ethylidene norbornadiene, dicyclopentadiene or trans 1,4-hexadiene, with ethylidene norbornadiene being often preferred. Such EPDM terpolymer rubbers, in general, are well known to those having skill in such art.
In the description of this invention, the term “phr” relates to parts by weight of an ingredient per 100 parts by weight of rubber.
The terms “rubber” and “elastomer”, “cure” and “vulcanize”, and “compound” and “composition” may be used interchangeably unless otherwise indicated.